Optical fibers are widely used within telecommunication systems for high-speed information transfer. A fiber unit, which could comprise a single optical fiber, or a bundle of optical fibers, is commonly installed into a protective optical conduit comprising optical fiber tubes, which have already been laid along the desired route, usually as a continuous span between convenient access points such as surface boxes or inspection chambers.
In this description, references to “cables” shall include where the context permits, individual optical fibers and fiber units as well as cables comprising such fibers and fiber units. “Conduits” shall include tubes and tube bores, but in the main refers to the route or path populated or to be populated by a fiber cable, and where the route comprises a number of tubes, the entire length of the route.
The conduits typically are made of plastic, each with a typical inner diameter of 3 to 6 mm or more, and are usually provided in a bundle comprising up to 24 or more tubes, which are held together within a protective outer sheath. Each fiber conduit tube can receive at least one fiber unit comprising one or more individual optical fibres. Large numbers of conduits—and bundles of conduits—are pre-installed across the access network and the distribution network between the local exchanges and the customer premises in a branching network system. With the move to pushing fiber links closer to customer premises (e.g. fiber to the premises “FTTP”), the conduits will further extend to and into commercial and residential premises. Indeed it is a fundamental part of the push to FTTP in e.g. the UK that substantially all the network comprises optical fiber, extending from the core network to as many end customers as possible. To achieve this, optical fiber installation needs to be speedy, cost- and effort-efficient.
Problems may arise during installation, which may result in the fiber unit not reaching the correct destination. During installation, the operator is usually presented with a large number of conduit tubes, which could result in a mistake in identifying the correct conduit, especially if the operator is working in adverse conditions down a manhole or in poor lighting. This may be so even where conduits are color-coded which helps to direct the operator to the correct conduit.
In an installation using the “blown fiber” method which is known from e.g. EP108590, a fiber unit is blown into a tube using pressurised air. If the air is applied into the wrong tube, the fiber unit will emerge in the wrong place (if at all). As it is seldom possible to know the exact length of the conduit route down which the fiber unit is being installed, the operator cannot even accurately know in a timely manner when something has gone wrong.
Currently, installing fiber units using the blown fibre method is relatively labor-intensive in requiring at least two operators: one situated at the head end of the conduit, where during installation air and the fiber unit is installed into the mouth of the conduit, and one at the remote end of the conduit, where air and the fiber unit emerges from the mouth of the conduit. The second remote end operator is required because the remote end is often some distance away—up to a kilometer or more—from the head end. The operator at the head end is therefore unable to know the status of the remote end during an installation—in particular whether air is flowing though the correct tube, and if/when the fiber unit has reached its destination—without a second operator located there.
Methods whereby a single operator at the head end of a conduit can detect the arrival of air, and subsequently, the fiber unit, at the remote end of the conduit are known. A number of them have been developed by the applicants, and include the method discussed in WO2007113549, where an acoustic signal is introduced into a bore within the sensing device which is attached to the far end of the tube so that the bore communicates with the bore of the tube. A phase shift in the acoustic signal resulting from air flowing through and along the bore of the sensing device is detected as being indicative of air flowing from the correct tube. Of course, if no change in the signal is detected, this is a strong indication that air is being fed down the wrong tube at the head end, or less likely, that the device has been fixed to the wrong tube at the far end. It has been found that while this method worked well in optimal conditions, changes in field conditions such as temperature affected performance reliability. Another approach is described in WO2007101975, where a substantially air-tight space is provided within the device housing. A rupture of the housing indicates that air is flowing out of the tube at the far end.